Vehicle size may have a bearing for real big ones but I have been in my 7.8mtr motorhome with a mate in his 7.5tonne converted DAF truck and we have had no problem parking up to sleep.
I have stopped overnight a couple of times on motorways services when I have just been too tired to carry on, Westmorland and Oxford services. Had to pay both times but both places gave you vouchers for food which reduced true cost to around a tenner I think. Thatās fine, I could have chosen to pull off the motorway and find somewhere if I really wanted to. Most if not all services now provide good hot showers as well.
I donāt find many problems
off grid stopping, I think itās more if you want to be in popular spots it may be you struggle